In a fuel system having hydraulically-actuated electronically controlled unit injectors (HEUI), high pressure hydraulic actuating fluid flows into a chamber, located within the injector, and pushes down on a plunger which pushes fuel out from a plunger cavity, and out the injector through a nozzle. A solenoid, located within the injector, controls when the high pressure actuating fluid is exposed to the plunger by moving a poppet valve. The amount of fuel injected is controlled by adjusting the duration the solenoid is on.
The viscosity of the actuating fluid effects both the amount of fuel delivered by the injector, and when the delivery process begins. For example, in cold temperatures the actuating fluid is thicker (more viscous) than at warm temperatures. Therefore, when an electrical signal is delivered to a solenoid, commanding the solenoid to deliver actuating fluid to the injector, the fluid flows at a slower rate into the chamber to push against the plunger. With the actuating fluid moving at a slower rate there is an increased delay before the injector begins delivering fuel. Furthermore, when the solenoid is again turned off to stop delivery of the fuel, the reduced flow rate of the actuating fluid results in less total fuel being injected between when the solenoid is turned on and off. Hence, with a high viscous actuating fluid seen at cold starting temperatures as compared to higher temperature operating conditions, the fuel is injection occurs later because of the slower rate of the fluid, and an inaccurate amount of fuel is delivered by the injectors and then starting/stopping, or "injection" fuel delivery shifts. Under these conditions, overall engine performance is adversely effected, resulting in incomplete combustion, low power, white smoke, etc. In addition, an engine may not be able to start if the injection does not occur at the correct time, with respect to the location of the piston within the cylinder chamber. Therefore, changes in temperature affect the ability to start an engine.
The viscosity of the actuating fluid is a function of the fluid type and the temperature of the fluid. In an operating engine, neither the type of fluid, nor the temperature is fixed. The fuel system may use a variety of actuation fluids. For example, a more viscous 15W40 engine oil or a less viscous OW20 engine oil may be used. Also the fuel system operates over a wide range of temperatures, e.g.,--40 degrees Celsius through 120 degrees Celsius.
The reduction in fuel delivery and delays in timing increase as the viscosity of the actuating fluid increases. If the changes in viscosity are not accounted for, the fuel delivery and timing may be incorrect making it difficult to start and run the engine especially at high viscosities encountered at cold temperatures. If the fuel delivery is too small, or is not delivered at the proper time, the engine may not start or be underpowered. If the fuel delivery is too large the engine structural capabilities may be exceeded, or excessive smoke produced and misfire may occur.
The present invention is directed to overcoming one or more of the problems identified above.